请问如何将ZK处理等待时的那个小gif的位置调到画面的中间？

.z-loading {  
	font-weight: bold;  
	padding:2px; 
	position:absolute;
	left:200px;
	top:200px;
	
}  
.z-loading-indicator {  
	color: gray;  
	border:0 none; 
	position:absolute;
	left:200px;
	top:200px; 
}  
.z-loading-icon{
	position: relative;	
}

<zk>  
	<style>  
	.z-loading {  
		font-weight: bold;  
		padding:2px; 
		position:absolute;
		left:200px;
		top:200px;
	
	}  
	.z-loading-indicator {  
		color: gray;  
		border:0 none; 
		position:absolute;
		left:200px;
		top:200px; 
	}  
	.z-loading-icon{
		position: relative;	
	}
	</style> 
	<button label="Test Progress" onClick="org.zkoss.lang.Threads.sleep(10000)"/>  
</zk>